From his past experience with HTML/CSS, he can already make a page, has VSCode, knows how to use VSCode to commit & sync. The big thing this time was that he had to learn a lot of JS and had to get much more proficient with CSS.
So my guidance was more of a set of hints as needed, for example "you want to boxes side by side, figure out how to do that using CSS" .. then he'd go away and talk to Gemini and ultimately Gemini would give him multiple approaches that he could try.
When it came to animation, I explained that there are many ways to animate (CSS, JS, etc) and guided him towards animated images. Basically, we "chatted" about a feature, I gave him some hints, then he went off and talked to Gemini for syntax and wrote the code. Many features he knew exactly what to do. If he wasn't sure, he'd have a discussion with me.
Basically, I was like a senior dev sounding board. He was the junior developer doing the work. Gemini was his Google/StackOverflow.
Kids dad here. When it was time to do images, I figured something like photoshop would be way too much for him. So I found a browser based sprite editor. The 32x32 grid simplified things and actually helped him.
Then one day I explained how the animation worked in the Piskel app (it had layers and frames). I came back an hour later and he had that flying snake that absolutely blew me away. He originally had it at 4 frames we gave him feedback that he needs more frames to be smooth. He upped it to 8.
The graphics is where had the least guidance from parents. We were focused on the code/logic aspects.
We had art lessons in primary school back in prehistoric times when I was that age (sixty years ago). Plenty of us could create more complex artwork than that.